Diseases of the teeth and of the periodontium, such as, for example, caries or periodontosis are nowadays usually diagnosed with X-ray-based imaging methods. Here, either conventional or so-called digital X-rays are used with projection methods or recently also three-dimensional (3D) methods, such as, for example, digital volume tomography (DVT), a type of X-ray computed tomography of the teeth and the viscerocranium.
However, for many dental diseases, a magnetic resonance imaging (MRI) examination would be the better alternative since it operates without ionizing radiation and enables a better representation of soft-tissue contrasts.
Magnetic resonance technology is a known technology which can be used to generate images of the interior of an object under examination. In simplified terms, for this purpose, the object under examination is positioned in a magnetic resonance device in a comparatively strong static, homogeneous basic magnetic field, also known as the B0 field with field strengths of 0.2 tesla to 7 tesla and more so that the nuclear spins of the object orient along the basic magnetic field. To trigger nuclear spin resonances, high-frequency excitation pulses (HF pulses) are irradiated into the object under examination, the triggered nuclear spin resonances are measured as so-called k-space data and used as the basis for the reconstruction of MR images or the determination of spectroscopy data. For spatial encoding of the measured data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The recorded measured data is digitized and stored in a k-space matrix as complex values. An associated MR image can be reconstructed from the k-space matrix populated with such values, for example by means of a multidimensional Fourier transformation.
However, an MRI system for dental medicine has to meet relatively high requirements for image quality, since the structures depicted are very small. In order, for example, to identify caries lesions, spatial resolutions of much smaller than a millimeter are required. At the same time, it is necessary for many questions (for example, an evaluation of the strength or porosity of the jaw bone) to represent bound protons, which in turn requires measurements with extremely short echo times, so-called “ultra-short TE”. However, this places high requirements on the steepness of the gradient fields since the phase difference of the spins only develops quickly enough with very steep gradients. In the case of a conventional MR system with a large field of view (FOV), this results in very high gradient amplitudes, which are either biologically impossible due to stimulation effects or would at least require an extremely expensive gradient system which is not economically viable for use in dental imaging.
Already known are intraoral receiver coils which are arranged in the patient's mouth and hence are very close to the target region and thus result in an improved signal-to-noise-ratio (SNR) compared to receiver coils arranged in more remote locations. Also known are dedicated head scanners for the head with a small field-of-view which only encompass the patient's head. However, nowadays these are mainly used for examinations of the brain.
Also known, for example from the article by Hennig et al.: “PatLoc: Imaging in non-bijective, curvilinear magnetic field gradients”, Proc. Intl. Soc. Magn. Res. 15, page 453, 2007 and the article by Parker et al.: “Multiple-Region Gradient Arrays for Extended Field of View, Increased Performance, and Reduced Nerve Stimulation in Magnetic Resonance Imaging”; MRM 56; pages 1251-1260, 2006, are MRI methods, which use non-unique field gradients of the gradient fields. Here, the gradient fields generated are not unique and can also be curved. The unique assignment of the signals to the positions is performed with the aid of the known illumination profiles of the coils used. These techniques are known by the name “PatLoc”. However, one drawback with these methods is the fact that there are positions at which the imaging cannot be performed at all or can only be performed with great difficulty, namely the zero positions of the gradients. Hitherto, techniques of this kind have been used for imaging of the brain, where for certain questions it is possible to dispense with precise imaging of the center of the brain and importance is above all attached to the high-resolution representation of the cerebral cortex (neocortex) such as is, for example, described in the aforementioned article by Hennig et al.
Also known from DE 10 2009 020 361 A1 is the use of non-unique gradients in the imaging the female breast.